US 20060176977 A1 Abstract A method for MIMO wireless communication comprises the steps of generating inner and outer codes based on channel state information available at a transmitter and concatenating different inner codes with different outer codes; and using the generated concatenated inner and outer codes for wireless communication. The inner and outer codes are based on channel phase information at the transmitter or channel feedback. High-performance trellis codes or block codes for use in cellular networks are illustratively described. The space-time trellis codes are generated by set partitioning on a plurality of classes of signal designs to generate a series of inner codes, each of the series of inner codes being optimized by channel phase feedback, and concatenating each inner code with a multiple trellis coded modulated outer code to provide a complete space-time trellis code as a cophase space-time trellis code.
Claims(26) 1. A method for wireless communication comprising:
generating inner and outer codes based on channel state information available at a transmitter; concatenating different inner codes with different outer codes; and using the generated concatenated inner and outer codes for wireless communication. 2. The method of 3. The method of 4. The method of 5. The method of 6. The method of 7. The method of 8. The method of 9. The method of 10. An apparatus for wireless communication comprising:
means for generating inner and outer codes based on channel state information available at a transmitter; means for concatenating different inner codes with different outer codes; and means for using the generated concatenated inner and outer codes for wireless communication. 11. The apparatus of 12. The apparatus of 13. The apparatus of 14. The apparatus of 15. The apparatus of 16. The apparatus of 17. The apparatus of 18. The apparatus of 19. A method for wireless closed-loop communication including at least one transmitter and receiver comprising:
generating quantized channel phase information for a fading channel at a receiver; feeding back the quantized channel phase information to the transmitter, where the inner code is generated by selection of one from a plurality of inner codes generated using the feedback of the quantized channel phase information; generating cophase space-time trellis codes by concatenating a selected inner code and an outer code at a transmitter, where the outer code is generated by multiple trellis code modulation; transmitting the generated concatenated inner and outer codes; receiving the generated concatenated inner and outer codes at the receiver where the quantized channel phase information was generated; and decoding the received concatenated inner and outer codes at the receiver. 20. The method of 21. The method of 22. The method of 23. An apparatus for wireless closed-loop communication including at least one transmitter and receiver comprising:
a transmitter with a transmit antenna; and a receiver with a receive antenna; where in the receiver quantized channel phase information for a fading channel between the receiver and transmitter is generated, and which is fed back to the transmitter, where in the transmitter a plurality of inner codes is generated and where one of plurality of inner codes is then selected using the feedback of the quantized channel phase information and where cophase space-time trellis codes are generated by concatenating a selected inner code and an outer code at a transmitter, where the outer code is generated by multiple trellis code modulation, and where the selected concatenated inner and outer codes are transmitted to the receiver where the received codes are decoded. 24. The apparatus of 25. The apparatus of 26. The apparatus of Description The present application is related to U.S. Provisional Patent Application, Ser. No. 60/644,075, filed on Jan. 14, 2005, which is incorporated herein by reference and to which priority is claimed pursuant to 35 USC 119. 1. Field of the Invention The invention relates to methods of design high-performance codes for the MIMO wireless communication systems and the apparatus which use them. 2. Description of the Prior Art For wireless communication systems, many codes have been devised to combat channel fading. However, many of these coding schemes were developed for single-transmit/receive-antenna systems, which cannot be directly applied to the recent multiple-input-multiple-output (MIMO) wireless systems. It is still an open problem to find good codes that can take full advantage of multiple transmit/receive antennas. In the recent years, some concatenated codes were presented for open-loop MIMO systems. Several block codes have been designed based on channel mean/covariance information at the transmitter. Block code is an error detection and/or correction code in which the encoded block consists of N symbols, containing K information symbols (K<N) and N-K redundant check symbols, such that most naturally occurring errors can be detected and/or corrected. More specifically, space-time trellis codes (STTCs) have been introduced to provide improved error performance for wireless systems using multiple transmit antennas. Space-time block codes operate on a block of input symbols producing a matrix output whose columns represent time and rows represent antennas. Unlike traditional single antenna block codes for the additive white Gaussian noise (AWGN) channel, most space-time block codes do not provide coding gain. Coding gain refers to the improvement in decibels (dB) that a particular code offers over other option. An improvement in coding gain can provide the designer with options such as reducing transmission power or bandwidth. Their key feature is the provision of full diversity with extremely low encoder/decoder complexity. Diversity is the property of being made up of two or more different elements, media, or methods. Diversity gain is the ratio of the signal field strength obtained by diversity combining to the signal strength obtained by a single path. Diversity gain is usually expressed in dB. In addition, they are optimal over all unitary codes with respect to the union bound on error probability. The best known codes for real constellations have been designed for a practical range of transmit antennas ( Space-time trellis codes operate on one input symbol at a time producing a sequence of vector symbols whose length represents antennas. Like traditional trellis coded modulation (TCM) for the single-antenna channel, space-time trellis codes provide coding gain. Since they also provide full diversity gain, their key advantage over space-time block codes is the provision of coding gain. Their disadvantage is that they are extremely difficult to design and require a computation ally intensive encoder and decoder. It was shown that, for an open-loop system, where only the receiver has full knowledge of the channel, the rank and determinant of the pair wise codeword difference matrix determine the coding gain of the corresponding space time trellis code. If the pair wise codeword difference matrix is full rank, full spatial diversity is obtained. Recently, the original space time trellis codes in have been enhanced by the super-orthogonal space time trellis codes (SOSTTC), and the super-quasi-orthogonal space time trellis codes (SQOSTTC). In these new trellis codes, a standard multiple trellis coded modulation (M-TCM) encoder serves as the outer encoder, while the space-time block codes (STBCs), or the quasi-orthogonal space time block codes (QOSTBCs), are used as building blocks for the inner codes. Multiple trellis coded modulation (MTCM) is trellis code in which each trellis branch corresponds to multiple symbol transmissions from each transmit antenna. The super orthogonal space time trellis code and super quasi-orthogonal space time trellis code enjoy full spatial diversity, higher coding gain, as well as simple implementation. The space-time coding schemes mentioned above do not exploit the channel knowledge at the transmitter. However, it is clear that with additional channel state information (CSI), the space-time transmission could be further improved. STBC beam forming schemes have been proposed based on the channel mean or covariance feedback. The schemes use precoding matrices which are constructed based on imperfect feedback of the mean or covariance of a complex Gaussian channel. Nevertheless, these schemes use complicated eigen-analysis to construct the optimal precoding matrices. In addition, the resulting beam forming matrix accomplishes optimal power loading, thus it normally incurs a high peak to average power ratio (PAPR) at the transmitter, which significantly increases the complexity and cost of the system. It may be difficult to implement these beam forming schemes in practical digital communication systems. Besides these STBC-based beam forming schemes, there are several other schemes that are based on traditional one-dimensional beam forming. Among them, a very promising scheme is the co-phase transmission (CPT) scheme. In cophase transmission, the relative channel phase information is uniformly quantized and sent back to the transmitter. On the transmit side, a rotation vector is applied to the transmission symbol. The rotation vector is constructed such that the signals from the different transmit antennas are added coherently at the receiver antenna, thus the receive signal to noise ratio (SNR) is maximized. The major advantage of cophase transmission is its easy implementation. With only a few feedback bits, significant performance improvement is attained. On the other hand, unlike the space time trellis code schemes, the original cophase transmission scheme does not provide any coding gain from the space-time transmission. In the prior art, the channel state information has not been used to design concatenated codes for multiple-input multiple-output (MIMO) communication systems. Furthermore, the design criterions in the prior art are developed based on channel mean/covariance information. However, what is needed is a coding scheme for MIMO wireless communication which is not subject to the foregoing limitations of the prior art. The illustrated embodiment of the invention generates optimal inner codes and outer codes for a concatenated code. Meanwhile a new design criterion is introduced based on channel phase information at the transmitter. The illustrated embodiment is thus directed to a method for code design in wireless communication systems and an apparatus which uses this code design. These new codes are constructed by concatenating different inner codes with different outer codes. Good inner and outer codes are obtained based on the channel state information available at the transmitter. We also introduce a design criterion for designing new codes based on channel phase information at the transmitter. This criterion can be used to design high-performance trellis codes and block codes. With the above design criterion and design method, we present some codes that are suitable for multiple-input-multiple-output (MIMO) wireless communication systems. These codes, including trellis codes and block codes, enjoy superior error performance and simple implementation. Based on the method and criterion, several high-performance codes are constructed by concatenating different inner codes with different outer codes. Good inner codes and outer codes are obtained based on the channel state information available at the transmitter and a new design criterion based on channel phase feedback. The fundamental principle is to use the channel state information at the transmitter to aid the code design. Compared to the prior art, the codes of the invention enjoy either superior error performance, or simpler implementation, or both. This invention is intended for use in MIMO wireless communication systems, and is useful for wireless communication system design, such as the next-generation cellular networks. What is disclosed is a new class of low-complexity space time trellis codes that combine the benefits of the coding gain from space-time coding and the maximum ratio combining gain from the channel phase feedback. To accomplish this goal, a new performance criterion is derived that takes the channel phase feedback into consideration. This new performance criterion is then used to perform set partitioning on several classes of signal designs. From the set partitioning results, we construct a series of inner codes. Each inner code is one of the most three favorable for one case of channel phase feedback. Finally, the newly designed inner codes are concatenated with a standard M-TCM outer code to obtain the complete space time trellis code. Since the proposed codes combine the advantage from both the space time trellis codes and the cophase transmission scheme, we name our new codes cophase space time trellis codes (CPSTTC). In the detailed description below we disclose the new performance criterion based on the channel phase feedback. We disclose the set partitioning of the different signal designs. Based on the set partitioning results, we demonstrate how to systemically design cophase space time trellis codes for a system with two transmitting antennas. We also provide a systematic approach to evaluate the coding gain for different cophase time trellis codes. Then we extend our method of designing cophase time trellis codes to systems with more than two transmit antennas. Finally, we present simulation results. While the apparatus and method has or will be described for the sake of grammatical fluidity with functional explanations, it is to be expressly understood that the claims, unless expressly formulated under 35 USC 112, are not to be construed as necessarily limited in any way by the construction of “means” or “steps” limitations, but are to be accorded the full scope of the meaning and equivalents of the definition provided by the claims under the judicial doctrine of equivalents, and in the case where the claims are expressly formulated under 35 USC 112 are to be accorded full statutory equivalents under 35 USC 112. The invention can be better visualized by turning now to the following drawings wherein like elements are referenced by like numerals. The invention and its various embodiments can now be better understood by turning to the following detailed description of the preferred embodiments which are presented as illustrated examples of the invention defined in the claims. It is expressly understood that the invention as defined by the claims may be broader than the illustrated embodiments described below. In the following disclosure: bold uppercase (lowercase) letters denote matrices (vectors); (▪)*, (▪) Space-time coding has been proposed recently for the MIMO wireless communication systems. Most of the proposed space-time coding schemes use the assumption that either no channel state information, or that the channel mean/covariance information is available at the transmitter A performance criterion is derived for the quasi-static fading channel. This design criterion is then used to construct a new class of space-time trellis codes. The proposed code construction is based on the concatenation of a standard M-TCM outer code with an inner code. The inner code is selected from a series of inner codes using the channel phase feedback. The series of inner codes are constructed based on the systematic set partitioning of several classes of space-time signal designs. Simulation results show significant performance improvement over the other space-time trellis codes in the prior art. In addition, the proposed coding scheme enjoys low peak-to-average power ratio, simple decoding, and easy implementation without complicated eigen-analysis. Performance Criterion Consider a system with M transmit antennas in the base station and a single receive antenna in the mobile station. We adopt a quasi-static Rayleigh fading model in the analysis in the illustrated embodiment, but the nature of channel fading is not a limitation of the invention. For a space-time codeword that lasts T symbol periods, the receive signal y is given by
where the components of the channel vector h=(h The object of the illustrated embodiment is to use the channel phase information at the transmitter Now we define the phase difference between two distinct transmit antennas as θij The above quantized phase feedback scheme is considered to be one of the most efficient feedback schemes in the art. The M=2 and L The design target of the illustrated embodiment is to minimize the conditional pair wise codeword error probability P In the above, P(C Using polar coordination, the upper bound is rewritten as:
The term P(h/{circumflex over (b)}) denotes the conditional probability density function of the channel coefficients. We have
and the integration regions are
To simplify this upper bound, we define
Combining the Jacobian of (v Then defining
The second integration term in (11) is simply given by ∫ At this stage, we assume that the union bound technique can be applied here and the worst-case pair wise error probability dominates the error performance. Using the union bound assumption, we obtain the following design criterion for constructing the optimal code C:
The remaining problem is to find codes that accomplish this minimum worst-case coding gain metric. Unfortunately, the coding gain metric in (12) is not in closed-form. For common phase shift keying (PSK) constellations such as BPSK, QPSK, 8PSK, and for a system with four or less transmit antennas, we find that the coding gain metric can be evaluated through simple numerical methods. Moreover, when M=2, the calculation of coding gain metric can be further simplified. For the M=2 case, only one parameter {circumflex over (b)} respectively. When M>2, we employ the following numerical algorithm to calculate coding gain metric: Algorithm Step 1. Through simple manipulations, the original coding gain metric is reformulated such that its integration region is bounded.
Step 3.
- A. If
$\uf603\frac{{\mathrm{CGM}}_{n}-{\mathrm{CGM}}_{n-1}}{{\mathrm{CGM}}_{n}}\uf604<\gamma ,$ then stop. Otherwise, go back to Step 3. The parameter γ denotes the convergence threshold. Heuristically, we set a constant threshold γ=0:01. - B. The convergence of this algorithm can be easily proved through basic calculus derivations. The above numerical calculation may be time consuming, and there might be many other algorithms that are simpler than the above algorithm. The invention thus explicitly includes other algorithms which determine the coding gain metric by other routines. However, the coding gain metric evaluation only needs to be carried out once in the code construction stage. It does not increase the complexity of the system implementation. Therefore, we determine that the above algorithm is sufficient, although further simplification of the coding gain metric calculation is contemplated as being within the scope of the invention. The illustrated algorithm is thus expressly not considered as a limitation of the invention.
- C. It is also worth mentioning that, because of the term σ
^{2}_{h}/σ^{2 }in (12), the receive signal-to-noise ratio (SNR) plays an important role in the trellis code design. To have an optimal code, the SNR should also be sent back to the transmitter**30**. Both the receiver**32**and transmitter**30**should maintain a large table which stores the different codes to be used at various SNRs. All these requirements increase implementation complexity. To avoid this, we design a series of codes for a constant σ^{2}_{h}/σ^{2 }and use it for all other values. Through a series of experiments, we find that σ^{2}_{h}/σ^{2}=10 is an appropriate constant. Below, we will use numerical results to demonstrate that this simplification has little effect on obtaining good codes. In fact, cophase space-time trellis codes (CPSTTC), which is the term for the codes generated by the invention, enjoy good performance at all SNRs.
Code Construction for Systems with Two Transmit Antennas Set Partitioning and Code Construction The system block diagram of cophase space-time trellis code, generally denoted by reference numeral A major goal in our code construction is to maintain the simple structure of the original super orthogonal space time trellis codes where c The second class of signal designs in which the symbols are transmitted simultaneously from different antennas is:
Consistent with prior usages we name this class of codeword matrices as ‘co-phase’ designs. Unlike the orthogonal designs, the co-phase designs do not provide full spatial diversity. However, the newly defined coding gain metric criterion is different from the original determinant and rank criterion previously used. Full spatial diversity is no longer a necessary condition for good performance. With the partial channel state information at the transmitter So far, the selection of the rotation angle φ in (16) and (17) has not been addressed. In general, we aim to avoid expanding the original signal constellation to maintain a simple implementation. Thus, φ=±π for BPSK and φ=kπ/2, 0≦k≦3 for QPSK, and so on. However, when the cardinality of channel feedback {circumflex over (b)} exceeds the cardinality of the corresponding N-PSK constellation, we have to exploit a wider range of rotation angles to achieve better performance. In this case, φ=2πk/max (2 In what follows, we perform set partitioning on the elements from A(c One important design parameter is the value of ψ in (13). An optimal strategy is to determine the value of ψ that can result in the smallest intra-coding gain metric for elements within certain subsets. In what follows, we provide several lemmas as guidelines for the optimal selection of ψ. Lemma 1. - A. Without loss of generality, the value of ψ can be restricted to the region ψ contained within the set [−π/2
^{L}, π/2^{L}) for L bits of feedback.
Lemma 2. - A. The value of ψ does not affect the intra-coding gain metric values for the elements from the orthogonal designs A(c
_{1}, c_{2}, φ).
Lemma 3. For N-PSK constellation, if ψ=0, the elements within
Now we are ready to perform set partitioning on the elements of A(c Based on the above set partitioning results, we systematically design a series of cophase space-time trellis codes. Straightforwardly, only subsets with relative small intra-CGMs are used to construct the inner codes. In addition, the parallel transitions diverging from or emerging into any state are from the same signal design with the same rotation angle. At the decoder In this disclosure, we focus on the design of rate one cophase space-time trellis code. The first example is a simple cophase space-time trellis code where the BPSK constellation is used and there is L=1 bit of feedback. A similar strategy works for more number of feedback bits as well. Using the case of L=2 bits feedback as an example, the set partitioning results for QPSK constellation are depicted in A key observation is that, our code construction uses a mixture of two classes of matrix designs, whereas the codes in the prior art use only one class. With a much larger set of elements available, our codes use only the elements that are more suitable for each channel situation, hence providing larger coding gains. B. Coding Gain Analysis Below, we provide a brief analysis of the coding gain for the above cophase space-time trellis codes 1) Error Events with Path Length of Two: We start with the simplest two-state trellis code in where D1 and D2 represent the difference matrices of the first and second transitions, respectively. From the trellis diagram in where Δc The corresponding coding gain metric value is 3.43×10 In the same way, we also calculate the worst-case coding gain metric values for the codes using QPSK constellation in 2) Error Events with Path Length of Three: We first examine the four-state codes in and c The corresponding coding gain metric value is 3.5×10 We repeat the above procedure for L=2 bits of feedback. It is observed that the worst-case intra-coding gain metric on parallel transitions is 2.4×10 Code Design for More than Two Transmit Antennas Consider now extending the general approach above for designing cophase space-time trellis codes Channel Phase Feedback and New Signal Designs When no channel state information is available at the transmitter where the parameters φ For the four-antenna cophase space-time trellis codes 1) 1-Bit Feedback Scenario: In this case, L The matrix in (24) is constructed in two steps. First, we remove the last row from the original 4×4 orthogonal design. Then we expand the first row into two rows, the signals on the second row are just rotated versions of the signals on the first row. By setting φ where φ 2) Two-Bit Feedback Scenario: There are several different ways to allocate the two feedback bits. A straightforward method is to use both bits to quantize θ where φ The last feedback scheme is to use one bit {circumflex over (b)} where L where φ 3) Three or More Bits Feedback Scenarios: - A. Similar to the two-bit feedback case, there are many different feedback strategies. When L
_{2}≦0, the signal design C(c_{1}, c_{2}, c_{3}, c_{4}, −2{circumflex over (b)}_{2}π/2L_{2}, φ_{2}, φ_{3}) is applicable. For L_{2}>0,L_{3}≦0 case, both C(c_{1}, c_{2}, c_{3}, c_{4}, −2{circumflex over (b)}_{2}π/2L_{2}, φ_{2}, φ_{3}) and D(c_{1}, c_{2}, c_{3}, c_{4}, −2{circumflex over (b)}_{2}π/2L_{2}, −2{circumflex over (b)}_{3}π/2L_{3}, φ_{3}) are applicable. When L_{2}>0, L_{3}=0, L_{43}>0, both C(c_{1}, c_{2}, c_{3}, c_{4}, −2{circumflex over (b)}_{2}π/2L_{2}, φ_{2}, φ_{3}) and E(c_{1}, c_{2}, c_{3}, c_{4}, −2{circumflex over (b)}_{2}π/2L_{2}, −2{circumflex over (b)}_{43}π/2L_{43}, φ_{3}) in (27) are applicable. When L_{2}>0,L_{3}>0,L4>0, the signal designs B(c_{1}, c_{2}, c_{3}, c_{4}, −2{circumflex over (b)}_{2}π/2L_{2}, −2{circumflex over (b)}_{3}π/2L_{3}, −2{circumflex over (b)}_{4}π/2L_{4}), C(c_{1}, c_{2}, c_{3}, c_{4}, −2{circumflex over (b)}_{2}π/2L_{2}, φ_{2}, φ_{3}) and D(c_{1}, c_{2}, c_{3}, c_{4},−2{circumflex over (b)}_{2}π/2L_{2}, −2{circumflex over (b)}_{3}π/2L_{3}, φ_{3}) are applicable. Finally, the signal design A(c_{1}, c_{2}, C_{3}, c_{4}, φ_{1}, φ_{2}, φ_{3}) is always a candidate for the inner code design since it does not rely on any channel phase feedback. - B. Using above examples, we have defined a series of signal designs. All of them accomplish a natural combination of co-phase designs and-orthogonal designs. The underlying principle is to combine two or more antennas into a ‘virtual’ antenna using co-phase designs, then combine the signals from the ‘virtual’ antenna with the signals from the rest of the antennas to form an orthogonal design. This new strategy can be easily extended to orthogonal designs or quasi-orthogonal designs for more than four transmit antennas. Finally, since no power loading is required in these codeword matrices, all these signal designs enjoy low peak-to-average power ratio.
C. Set Partitioning and Code Construction - D. We have presented a series of feedback bit allocation schemes and corresponding signal designs. Obviously, some of the signal designs provide better coding gain compared to the others. To pick the right combination, we adopt the same principle that we have used to construct the codes for two transmit antennas. We carry out set partitioning on all the candidate signal designs. Based on the results, only the ones that provide small intra-coding gain metric are used in the inner codes.
- E. For the L≦6 cases, the set partitioning results for BPSK constellation is provided in
FIG. 7 . For illustration only three signal designs are presented that provide the smallest intra-coding gain metric. A significant byproduct of the above signal designs is that they can also be used in non-trellis coded systems. Using the intra-coding gain metric values at the root level of the partition tree, the receiver**32**can pick the best signal designs, as well as the corresponding feedback scheme. Without the outer M-TCM encoder**18**, these signal designs can be readily used as space time block codes. A major contribution of these new block codes is that the coding scheme is jointly defined with the channel phase feedback scheme, which is not addressed by the precoding schemes in the prior art. Furthermore, the preferred symbol-by-symbol decoding algorithm is applicable for all these new block codes. - F. The trellis code construction based on the set partitioning results is straightforward. For any given trellis, we assign the signal designs with the smallest intra-coding gain metric on the different states using the Ungerboeck rules, which are:
- U1 Members of the same largest partition are assigned to parallel transitions.
- U2 Members of the next larger partition are assigned to “adjacent” transitions, i.e. transitions stemming from, or merging into the same node.
- U3 All the signals are used equally often.
- G. In the meantime, the bit allocation of the feedback scheme is also determined. As an example,
FIGS. 8 *a*and**8***b*depicts the two-state and four-state cophase space-time trellis codes**10**respectively for transmitting 1 bit/s/Hz using BPSK for L≦·6 cases. The bit assignment on the feedback channel is also included inFIGS. 8 *a*and**8***b.*
Numerical Simulations Consider the performance of the cophase space-time trellis codes In the first simulation, the system consists of two transmit antennas and one receive antenna, and each frame consists of 130 transmissions. In the second simulation, we examine a system with four transmit antennas and one receive antenna. For the 4-TCM codes above, each frame consists of 132 transmissions. We also perform numerical simulations for the two-state codes in It can now be appreciated that we have constructed a new class of codes called co-phase space time trellis codes. The proposed coding scheme is based on a practical assumption that only a few quantized channel phase feedback bits are available at the transmitter Numerical simulations have demonstrated significant gain over the most recent open-loop transmission schemes as well as the close-loop cophase transmission schemes. In addition, the proposed coding scheme enjoys low peak-to-average power ratio, simple decoding, and easy implementation without complicated eigen-analysis. The invention includes within its cope the extension of this idea to super quasi-orthogonal space time trellis codes. Preliminary results show that full rate cophase space-time trellis codes based on quasi-orthogonal designs accomplish significant performance gain compared to the prior art super quasi-orthogonal space time trellis codes. Many alterations and modifications may be made by those having ordinary skill in the art without departing from the spirit and scope of the invention. Therefore, it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the invention as defined by the following invention and its various embodiments. Therefore, it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the invention as defined by the following claims. For example, notwithstanding the fact that the elements of a claim are set forth below in a certain combination, it must be expressly understood that the invention includes other combinations of fewer, more or different elements, which are disclosed in above even when not initially claimed in such combinations. A teaching that two elements are combined in a claimed combination is further to be understood as also allowing for a claimed combination in which the two elements are not combined with each other, but may be used alone or combined in other combinations. The excision of any disclosed element of the invention is explicitly contemplated as within the scope of the invention. The words used in this specification to describe the invention and its various embodiments are to be understood not only in the sense of their commonly defined meanings, but to include by special definition in this specification structure, material or acts beyond the scope of the commonly defined meanings. Thus if an element can be understood in the context of this specification as including more than one meaning, then its use in a claim must be understood as being generic to all possible meanings supported by the specification and by the word itself. The definitions of the words or elements of the following claims are, therefore, defined in this specification to include not only the combination of elements which are literally set forth, but all equivalent structure, material or acts for performing substantially the same function in substantially the same way to obtain substantially the same result. In this sense it is therefore contemplated that an equivalent substitution of two or more elements may be made for any one of the elements in the claims below or that a single element may be substituted for two or more elements in a claim. Although elements may be described above as acting in certain combinations and even initially claimed as such, it is to be expressly understood that one or more elements from a claimed combination can in some cases be excised from the combination and that the claimed combination may be directed to a subcombination or variation of a subcombination. Insubstantial changes from the claimed subject matter as viewed by a person with ordinary skill in the art, now known or later devised, are expressly contemplated as being equivalently within the scope of the claims. Therefore, obvious substitutions now or later known to one with ordinary skill in the art are defined to be within the scope of the defined elements. The claims are thus to be understood to include what is specifically illustrated and described above, what is conceptionally equivalent, what can be obviously substituted and also what essentially incorporates the essential idea of the invention. Referenced by
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